1. Field of the Invention
The invention relates to a radar system that is mounted on a vehicle, or the like and detects an obstacle and/or a preceding vehicle ahead and to a detection method.
2. Description of Related Art
A vehicle has already been available, on which a radar system is mounted to detect obstacles and/or preceding vehicles ahead of the vehicle and determine the relative distance to the obstacles and/or the preceding vehicles.
The radar system is mounted in a front portion of the vehicle and a transmitting antenna of the radar system outputs a transmitted wave ahead of the vehicle. When the transmitted wave hits the obstacles, the preceding vehicles, etc. and is reflected therefrom, the reflected wave is received by a receiving antenna of the radar system. The reflection source, such as the preceding vehicles and the obstacles, is herein collectively referred to as the target.
After the reflected wave is received by the receiving antenna, an arithmetic processing unit in the radar system calculates the relative distance between the vehicle and the target based on the time lag (delay time) between the transmitted wave and the reflected wave. In addition, the arithmetic processing unit is connected to the warning device and when, based on the result of calculation of the relative distance, it is determined that the target can approach the vehicle and collide with the vehicle, the arithmetic processing unit sends a warning command to the warning device. The warning device that has received the warning command sounds the alarm or displays a warning message to prompt the driver to decelerate the vehicle and/or avoid the obstacles and/or the preceding vehicles.
For the purpose of sending a warning command, the arithmetic processing unit determines the height of the target. When the height of the target is low and the vehicle can easily run over the target, the vehicle would not collide with the target and therefore, there is no need to send the warning command to the warning device. Thus, when a target is detected, the height of the target is determined and it is determined whether it is necessary to send a warning command.
As a method of determining the height of the target, a method that utilizes the intensity (amplitude) of the reflected wave has already been available. It is known that the intensity of the reflected wave varies depending on the relative distance between the vehicle and the target and that the trend of the variation depends on the height of the target. Japanese Patent Application Publication No. 2008-122391 (JP-A-2008-122391) describes that a curve of the intensity of the reflected wave in relation to the relative distance between a radar system and a target is obtained through experiments or the like, the curve is obtained for each height of the target, and the curves for the respective heights are stored in a memory of the radar system. The curve of the intensity of the reflected wave that is detected by the radar system while the vehicle is driven and the plurality of curves for the respective heights that are stored in the memory are compared with each other (subjected to the pattern matching), the curve that is minimum in the difference from the curve of the reflected wave that is detected by the radar system is extracted from among the curves for the respective heights stored in the memory, and the height of the target corresponding to the extracted curve is assumed to be the height of the target that is detected by the radar system.
For the purpose of improving safety, it is desirable that the target be detected as early as possible, that is, while the target is still distant from the vehicle. This is accomplished by increasing the detectable range of the radar system by increasing the power of the oscillator that generates the transmitted wave. By increasing the power of the oscillator, it becomes possible to increase the detectable range that has been approximately 50 m, to approximately 150 m.
Meanwhile, because of the propagation characteristics of the transmitted wave, the detectable range of the radar system is increased not only in the travel direction but also in the horizontal direction and the vertical direction. As a result, the transmitted wave is reflected by the upper structures, such as the road signs and overpasses, that were not reached by the transmitted wave. Because there is no fear that the upper structures hit the vehicle, it should be avoided that a warning command is sent in response to detection of the upper structures. For this reason, after the target is detected, it is necessary to determine the height of the target to determine whether the target is an upper structure or a preceding vehicle, and to determine whether it is required to send a warning command.
When the detectable range is increased, however, the reflected wave that is received by the receiving antenna contains much noise and as a result, although the height of a preceding vehicle and the height of an upper structure differ from each other, the difference between the intensities of the reflected waves therefrom becomes unclear. The waveform of the intensity of the reflected wave from an upper structure and the waveform of the intensity of the reflected wave from a preceding vehicle when a conventional radar system is used are shown in FIG. 12 and FIG. 13, respectively. As apparent from these diagrams, there was no clear difference between an upper structure and a preceding vehicle in the case of the conventional technology and it has been difficult to determine whether the target is a preceding vehicle or an upper structure, based on the intensity of the reflected wave.